Both Canon A3100 IS and Canon S120 have 12.0 MP resolution sensors but Canon A3100 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Canon S120's sensor is 1/1.7-inch (7.44 x 5.58 mm ).
Since Canon S120's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Canon A3100 IS.
Below you can see the A3100 IS and S120 sensor size comparison.
As seen above, Canon S120 has a 1.5x Larger sensor area than Canon A3100 IS. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.